Toxicology – Antidote: N-Acetylcysteine (NAC)

Indications

N-acetylcysteine is the primary antidote for acetaminophen (APAP) poisoning and can be given orally or intravenously. It may also be considered in some other toxicologic conditions involving oxidative stress or glutathione depletion.

For a known single acute acetaminophen ingestion, obtain a serum level at least 4 hours after ingestion and interpret it using the Rumack–Matthew nomogram. NAC is indicated when the level falls above the treatment threshold.

Treatment should also be strongly considered when:

• The time of ingestion is unknown
• The acetaminophen level is detectable with uncertain timing
• Liver enzymes, particularly AST/ALT, are elevated in a patient with possible acetaminophen toxicity

If the acetaminophen concentration is undetectable and liver enzymes are normal, clinically important toxicity is less likely.

Mechanism of Action

NAC helps protect the liver through several mechanisms:

• Replenishes glutathione, which detoxifies the reactive acetaminophen metabolite NAPQI
• Can act as a glutathione substitute and antioxidant
• Enhances sulfation pathways involved in acetaminophen metabolism
• After liver injury has already occurred, it may improve tissue oxygen delivery and reduce oxidative and inflammatory damage

Administration

Both oral and IV NAC are effective. IV therapy is often preferred when oral treatment cannot be tolerated or in patients with severe hepatic injury.

Common treatment protocols include:

• Oral NAC: loading dose followed by repeated maintenance doses over an extended course
• IV NAC: typically administered as a multi-stage infusion over approximately 21 hours

Treatment may need to continue beyond the standard protocol if acetaminophen remains detectable or liver injury is still progressing.

Monitoring

During treatment, follow:

• Acetaminophen concentration
• AST and ALT
• INR/coagulation studies
• Renal function
• Clinical signs of hepatic failure

NAC is generally continued until acetaminophen is no longer detectable and hepatic injury is clearly improving.

Adverse Effects

IV NAC can cause anaphylactoid reactions, including flushing, rash, wheezing, or hypotension. These are usually managed by temporarily slowing or stopping the infusion and providing symptomatic treatment before restarting when appropriate.

Key Points

• NAC is most effective when started early, but it can still provide benefit even after liver injury has developed.
• Do not delay treatment when significant acetaminophen poisoning is strongly suspected.
• The Rumack–Matthew nomogram applies only to a known, single, acute ingestion with a known time.
• Continued NAC may be necessary when liver enzymes remain markedly abnormal or acetaminophen is still detectable.